1. Field of the Invention
The present invention relates to an optical position detecting method and an apparatus for projecting incident optical beams on a measured surface of an object having two types of reflecting regions which are different in reflectance from each other, and of receiving reflected optical beams reflected by the reflecting regions on a photoelectric conversion element of at least a one dimensional arrangement, to thereby detect the projected position of the optical beams on the basis of a light signal received from the photoelectric conversion element.
2. Description of the Prior Art
In general, a contact exposure method, a proximity exposure method or the like is employed as a method of transferring a mask pattern of a photomask onto a substrate. In the proximity exposure method, the photomask and the substrate must be precisely arranged to be separated by a constant space of 20 to 50 .mu.m, for example.
Thus, there have been proposed a number of devices for measuring the space between the photomask and the substrate. FIG. I illustrates a conventional position detecting apparatus for a proximity exposure system. As shown in FIG. 1, the conventional position detecting apparatus is adapted to project incident optical beams B.sub.1 on the surface of an objective photomask 101 and to receive optical beams B.sub.2 reflected by the photomask 101 onto a semiconductor position detector 107 of one-dimensional or two-dimensional arrangement. It is thus possible; to calculate the surface position of the photomask 101 on the basis of of received light signals I.sub.A and I.sub.B thereof. Assuming that symbol L denotes the distance between electrodes A and-8 and the symbol K denotes the distance between the electrode A and the position receiving the reflected optical beams B.sub.2, the semiconductor position detector 107 outputs current values, in accordance with the following equations, from the respective electrodes A and B: ##EQU1## where the symbol I.sub.O denotes the light intensity of the reflected optical beams B.sub.2.
Further, an arithmetic circuit 110 outputs the following ratio P.sub.X of the current value I.sub.A to the current value I.sub.B as a signal corresponding to the position X of the surface to be measured: EQU P.sub.X =(L-X)-1 . . . (2)
The known position detecting apparatus is not satisfactory, however, because it is not sufficiently accurate. The reasons are as follows; position detecting apparatus cannot satisfy the requirement for higher accuracy. The reason for this is as follows:
The photomask comprises a fine mask pattern of a thin film provided on the surface of a glass plate, which mask pattern is formed by a film of chromium, for example, having a reflectance different from that of the glass plate.
Particularly, the reflecting surface of the photomask to be subjected to measurement is formed by the mask pattern portions and the exposed portions of the glass plate. As a result, two types of reflecting regions, which are different in reflectance from each other, are mixed in the reflecting surface of the photomask.
Therefore, an image formed by the reflected optical beams B.sub.2 consists of regions G formed by an optical subbeam reflected by the glass plate portions and regions P formed by an optical subbeam reflected by the mask pattern portions, as shown in FIG. 2, and the conceptual light intensity distribution thereof is as shown in FIG. 3. Due to the variation in reflectivity of the two portions, the center-of-gravity position of the light intensity distribution varies with a change in the ratio of the glass plate portions to the mask pattern portions existing on the reflecting surface. This has the effect of changing the distance X. Assuming that a diameter D of the received optical beams B.sub.2 is 50 .mu.m and the ratio of reflectance of the glass plate portions to that of the mask pattern portions is 0.2, for example, an error of +10 .mu.m may be produced in the detected position of the measured surface.